All-optical control and multiplexed readout of multiple superconducting qubits

Abstract

Superconducting quantum circuits operate at millikelvin temperatures, typically requiring independent microwave cables for each qubit for connecting room-temperature control and readout electronics. However, scaling to large-scale processors hosting hundreds of qubits faces a severe input/output (I/O) bottleneck, as the dense cable arrays impose prohibitive constraints on physical footprint, thermal load, wiring complexity, and cost. Here we demonstrate a complete optical I/O architecture for superconducting quantum circuits, in which all control and readout signals are transmitted exclusively via optical photons. Employing a broadband traveling-wave Brillouin microwave-to-optical transducer, we achieve simultaneous frequency-multiplexed optical readout of two qubits. Combined with fiber-integrated photodiode arrays for control signal delivery, this closed-loop optical I/O introduces no measurable degradation to qubit coherence times, with an optically driven single-qubit gate fidelity showing only a 0.19% reduction relative to standard microwave operation. These results establish optical interconnects as a viable path toward large-scale superconducting quantum processors, and open the possibility of networking multiple superconducting quantum computers housed in separate dilution refrigerators through a centralized room-temperature control infrastructure.

Figures (4)

• Figure 1: The concept of cable-free optical I/O architecture for a superconducting quantum system.(a) Schematic of the all-optical closed-loop link of qubit drive and readout. (b) Schematic of the traveling-wave Brillouin microwave-to-optical (M2O) transducer. (c) Photograph of the packaged M2O transducer. The microwave port is wire-bonded to a printed circuit board. The optical fiber is fixed onto the sample using fiber optic adhesive (black). The optical fiber end is located above the grating coupler (red). (d) Photograph of the structure of the M2O transducer.
• Figure 2: Characterization of the broadband traveling-wave M2O transducer.(a) Schematic of the conversion process between the microwave mode ($\hat{a}_\text{e}$) and the optical mode ($\hat{b}_\text{s}$). $\kappa_{e},\kappa_m$, and $\kappa_o$ represent the coupling rates of each mode to the external environment. $n_{in(out)}$ denotes the photon number of the input (output) field. (b) Reflected optical spectrum from the transducer at room temperature for different input microwave powers (from $-15\,\text{dBm}$ to $+15\,\text{dBm}$ in $5\,\text{dBm}$ increments), showing both reflected pump ($1550.00\,\text{nm}$, green line) and the generated Stokes signal ($1500.07\,\text{nm}$, red line). (c) Heterodyne beat note power as a function of input microwave power, with a fixed pump ($100\,\text{mW}, 1550\,\text{nm}$). The reflected light is amplified to $5\,\text{dBm}$ using an Erbium-Doped Fiber Amplifier (EDFA) before detection by a high-speed photodetector (HPD) to obtain the beat note signal. (d) Microwave spectrum of the beat note signal for a fixed pump ($100\,\text{mW}, 1550\,\text{nm}$) before and after cooling in the dilution refrigerator. The M2O transducer is mounted on the $3.3\,\text{K}$ stage. (e) Microwave spectrum of the beat note signal at $3.3\,\text{K}$ for different pump wavelengths ($100\,\text{mW}$) swept from $1534\,\text{nm}$ to $1570\,\text{nm}$ in $2\,\text{nm}$ increments.
• Figure 3: Simultaneous readout of multiple qubits via the M2O transducer.(a) Diagram of the simultaneous readout. The readout signals of two transmon qubits ($\omega_{r,1},\omega_{r,2} =7.509\,\mathrm{GHz}, 7.584\,\mathrm{GHz}$) are upconverted to $8.743\,\mathrm{GHz},8.818\,\mathrm{GHz}$ by a Josephson parametric converter (JPC) using a $1.234\,\mathrm{GHz}$ microwave pump. These microwave readout signals are then simultaneously converted into different optical signals by the transducer with two pump lights ($\lambda_{p,1}=1550.00\,\mathrm{nm},\lambda_{p,2}=1560.95\,\mathrm{nm}$). (b) and (c) IQ scatter diagrams of the single-qubit optical readout of qubit $\mathrm{Q}_1$, averaged over 1 and 100 measurements, respectively. The readout pulse length is $10\,\mu \mathrm{s}$. (d) Histogram of the I quadrature averaged over 100 measurements. (e) Optical readout of a power Rabi experiment on transmon qubit $\mathrm{Q}_1$. The optical pump wavelength and the corresponding microwave frequency are tuned to enable an optical readout over a bandwidth of $200\,\mathrm{MHz}$. The color map represents the amplitude of the beat note signal. (f) Microwave spectrum of the beat note signal when the transducer is pumped simultaneously by two pumps ($\lambda_1=1550.00\,\mathrm{nm},\lambda_2=1560.95\,\mathrm{nm}$). (g) Optical and microwave pulse sequences for the simultaneous optical readout. (h) Simultaneous optical readout of the power Rabi experiment for two qubits, with each data point averaged over $5\times 10^4$ measurements.
• Figure 4: All-optical control and readout of superconducting qubits.(a) Comparison between standard microwave I/O and all-optical I/O. (b) Random Benchmarking experiment under optical drive. Both the qubit control and readout pulses are generated via the optical downlink. (c) Single-qubit gate fidelity under optical and microwave drive. All gates are implemented using $120$ ns Gaussian pulses. (d), (e), (f) Measurements of $T_1$, $T_2$, and $T_\mathrm{2E}$ of the superconducting qubit ($\mathrm{Q}_2$) under different combinations of drive and readout methods: optical drive with optical readout (purple), microwave drive with optical readout (blue), optical drive with microwave readout (red), and microwave drive with microwave readout (orange). $P(\left|g\right\rangle)$ and $P(\left|e\right\rangle)$ are the measured populations of the qubit in the ground and excited states, respectively. In the Ramsey experiment, the two $\pi/2$ pulses are applied with a detuning of $0.20$ MHz relative to the qubit frequency for optical readout and $0.42$ MHz for microwave readout.